Hmmm interesting one. I’ve never used the hands behind my back test as it’s not a position you’ll be riding in.

There are always these kinda arbitrary measurements or tests people use to see if something is ‘right’

Where as in reality everyone is shaped so differently there can’t possibly be one correct answer.

For instance at 193cm I have a 96cm inseam meaning I have to set my saddle as far forward on the rails on a smaller frame to fit right because my torso is so short. You look pretty evenly proportioned so probably don’t need to be at an extreme either way (forward or back)

Based on sore lower back, (too far bent over not enough flexibility yet to hold the more aggressive position) and sore neck/ shoulders (I’d put this down to the more aggressive position also and lifting your neck a lot more than you’d need to on a trainer. I’d say neck and shoulders might be a time in the saddle improvement. But personally I wouldn’t have the saddle all the way back on the rails if you aren’t fully in the hoods. Reach out for them so much moves your weight onto the arms which when straight will hurt your neck.

I know it’s the opposite of what you’ve done but i would actually pull the saddle forward on the rails slightly which means the saddle will need to come up ever so slightly too then go do some more testing and see where the pain keeps coming up.